CAMPBELL -- A 16-year-old San Jose boy was killed and a homeless man was arrested after a stabbing at a popular city park Sunday night, but it's not clear to investigators what sparked the deadly confrontation.

Both the slain teen and the suspect were described by their friends as peaceful people for whom starting a fight would be out of character. The possibility that the stabbing might have been in self-defense has not been ruled out.

Whatever the cause, two high school campuses in the West Valley were reeling from the loss of Ivan Diaz Amezcua, a junior at Boynton High School who recently transferred from Westmont High School. Boynton is an alternative school with an enrollment of about 180 students.

"Our hearts go out to the family and friends of this student," said Mike Posey, a spokesman for the Campbell Union High School District.

Posey said additional counseling, along with grief services, were being offered Monday for students and staff at both campuses.

Meanwhile, 24-year-old Todd Tharp, who police described as a transient, was arrested on suspicion of murder in Diaz's death, the city's first homicide of the year, after two were recorded in 2013.

Tharp was initially treated as a victim when Campbell police were called to John D. Morgan Park around 8:25 p.m. Sunday for reports of an injured man. Officers found Tharp near a baseball field and noted he had injuries suggesting he had been in a serious fight.

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As they investigated the call, police learned a teenage boy was taken to the emergency room at Valley Medical Center and was suffering from a stab wound. He was rushed into surgery but was pronounced dead just before 3 a.m. Monday, nearly seven hours after police believe he was stabbed.

Tharp was arrested in connection with the death after being treated for his injuries. Police recovered a 6-inch hunting knife believed to have been used in the stabbing.

Police believe Diaz was with two or three friends when the altercation took place, said Campbell police Capt. Dave Carmichael. Detectives are determining how many other people were involved and are looking into all possibilities in the case, including a self-defense angle for Tharp.

"We're still talking to everyone involved and trying to determine what led up to the altercation," Carmichael said. "We don't have anything to say they did know each other, but we can't rule that out either."

According to KTVU, family members said Diaz had headed to the park after leaving a barbecue, and he was described as a peaceful person. But the same went for Tharp: Friends described him as gentle, sociable and non-confrontational.

Tharp said he was only protecting himself. In a jailhouse interview, he told NBC Bay Area that he used a knife to ward off multiple attackers who knocked him to the ground and kicked him repeatedly in the head.

Monday afternoon, there were no evident signs of a struggle near the baseball field at John D. Morgan Park. Police say the fight happened by the bleachers near the third base side, which is not visible from the park area.

There were two homicides last year in Campbell: the shooting death in March of an 18-year-old man near the community center and the murder-suicide of an 89-year-old man and his 92-year-old wife.

Anyone with information about the case is asked to contact Campbell police Sgt. Gary Berg at 408-871-5190.